To facilitate discussion, much of the ensuing description will largely be with reference to styrene production and purification. However, similar principles and considerations apply in the case of other analogous vinylaromatic monomers.
Styrene monomer is usually produced commercially by the dehydrogenation of ethylbenzene in a vapor phase, fixed catalyst-bed reactor. Each pass through the reactor converts about 60 to 70% of the ethylbenzene to styrene. Certain impurities are produced in the styrene reactor--mainly, benzene and toluene. Other impurities are present in the incoming ethylbenzene feed. These are mainly xylenes, cumene, .alpha.-methylstyrene, and polyethylbenzenes.
Distillation is used to purify the styrene monomer to customer specifications (usually 99.9+wt %). The ethylbenzene is removed for recycle to the styrene reactor. Benzene is recycled to the Ethylbenzene Unit in which ethylbenzene is produced by ethylation of benzene. Toluene is recovered as a salable product. The other impurities are used as fuel for the process. Typically a styrene plant will use one of two commercially available distillation configurations such as illustrated in FIGS. 1 and 2. The primary differences in the two systems is whether the fresh feed is introduced into the BT Column or into the EB Recycle Column.
The styrene monomer is a reactive chemical used to produce a wide range of polymers and rubbers. This reactivity makes distillation of styrene monomer difficult because the styrene tends to polymerize in the distillation column. Any styrene monomer which polymerizes will be removed with the heavies and used as fuel. This polymerization represents a serious economic penalty when valuable product styrene is converted to a waste fuel. If the polymerization reaction goes too far, the distillation system may become fouled with polymer thereby decreasing efficiency, or may even plug completely with hard polymers thereby damaging equipment. Since the rate of styrene polymerization increases with increase in temperature, conventional practice involves operating the distillation columns of commercial styrene plants at low pressures to reduce boiling temperatures and thereby reduce the extent of adverse polymerization. Table 1 shows typical temperatures and pressures of a modern styrene plant.
TABLE 1 ______________________________________ Styrene Distillation Pressure and Temperature Relationships Distillation Column Pressure, Temperature, Column Portion psia Degrees F. ______________________________________ BT Column Top 4.0000 145 Bottom 4.5029 207 EB Recycle Top 1.4500 162 Bottom 2.9780 206 Styrene Finishing Top 1.1000 159 Bottom 1.2159 192 Styrene Recovery Top 2.5145 159 Bottom 3.9458 289 ______________________________________
In addition to temperature control, another method is used to reduce polymerization. So far as is known, all commercial plants add a polymerization inhibitor to the styrene distillation train. The polymerization inhibitor shifts the polymerization rate curve to higher temperatures and thus reduces the amount of polymerization occurring at operating temperatures. A number of inhibitors are available with varying degrees of effectiveness.
Since the primary defense against polymerization is distillation at reduced temperature achieved by use of low pressure (vacuum), standard commercial practice is to operate styrene plants at as low a pressure and temperature as possible. Vacuum distillation columns are usually very large. The largest column in any styrene plant is the Ethylbenzene (EB) Recycle Column.
Typically, this column is close to 200 feet high and 16 to 36 feet in diameter.
A number of factors have been used in arriving at the size and type of columns installed in a commercial styrene distillation facility. One factor is column height. A distillation column is designed to separate chemicals by repeated boiling and condensing of the chemical mixture. The lighter or lower boiling point temperature chemicals will concentrate in the vapor and leave the column overhead and the heavier chemicals will concentrate in the liquid and leave the column bottom. The closer together the boiling point of the chemicals to be separated, the more difficult is the separation. The measurement of this degree of separability is called relative volatility. Higher relative volatility means easier separation.
Ethylbenzene and styrene have close boiling points. This means the mixture must be boiled and condensed many times to achieve a separation. The amount of separation achieved if the mixture was heated one time and allowed to reach equilibrium between the vapor and the liquid is called one theoretical stage.
In a distillation column, vapor and liquid phases flow countercurrently. The column is fitted with trays or packing to maximize the contact between the two phases. At the bottom of the column, the liquid mixture is heated to send vapor up through the column. At the top of the column, vapor is condensed and sent back down the column as liquid. The liquid sent back to the top of the column is called reflux. The term "reflux ratio" is usually defined as the quantity of liquid returned to the column divided by the amount of liquid product taken from the overhead. The number of theoretical stages required for any given separation of a mixture of two or more chemicals is a function of reflux ratio. FIG. 11 is the Gilliland correlation which demonstrates this relationship of theoretical stages to reflux ratio. The curve for any given separation is asymptotic on one axis to the minimum number of stages at total reflux, and on the other axis to the minimum reflux required at an infinite number of stages. Therefore by setting a reflux ratio, the number of theoretical trays required in a distillation column may be determined. In the actual column, the number of stages determines the number of trays required in a tray column or the height of packing needed in a packed column. The number of trays or packing height in turn sets the column height. Consequently, the height of a distillation column may be determined at least in part by defining the separation required and the reflux ratio to be employed.
Another factor used in arriving at column size and type is column diameter. The diameter of a distillation column is determined by internal flow rates of liquid and vapors. Vacuum distillation towers are sized for the vapor volumetric flow rate (ft.sup.3 /hr or m.sup.3 /sec). Liquid rates typically do not determine vacuum tower diameter. At maximum vapor loading for any distillation column, liquid begins to be entrained in the vapor. If the vapor rate is increased, the liquid will not be able to flow freely through the column and the column will fill with liquid. This condition is called flooding.
Reflux ratio affects the vapor loading. Every pound of liquid sent back to the column as reflux must first have traveled through the tower and out the overhead as a vapor. Therefore, higher reflux results in higher vapor loading.
In designing a distillation column, a decision must be made whether to invest in tower height or in tower diameter. Operation at the minimum reflux ratio would allow the minimum vapor flow through a column and, therefore, the minimum column diameter. At the minimum reflux ratio the tower must be infinitely tall to make the given separation. Obviously the tower must be designed with a higher than minimum reflux ratio to reduce the tower height to a practical size. In the case of a high capacity vacuum tower, such as those used in the usual current styrene process, the practical limit for tower diameters is being approached. To minimize the diameter, reflux ratios near the minimum are being used.
Still another factor tending strongly to increase column size is large plant capacity and throughput. In practice, large diameter vacuum columns are required in styrene distillation facilities in order to process the large capacity of modern plants and maintain operating temperatures low enough to limit styrene polymerization to an acceptable level. However, these large diameter vacuum towers have certain problems inherent with their size.
Most new plants use structural packing in the B/T column and EB Recycle Column. A few have installed structural packing in the Styrene Recovery Column (not to be confused with the finishing column employed in preferred embodiments of this invention described hereinafter). Structural packing has a much lower pressure drop per theoretical stage than a trayed column. Since the pressure drop across the total column is lower, the pressure on the bottom of the column (maximum pressure location) is lower. A lower bottom pressure creates a lower bottom temperature and; therefore, results in less polymerization.
Sizing a structural packing vacuum column requires consideration of another factor, viz., minimum liquid loading. Structural packing requires a minimum liquid loading in gallons per minute or pounds of liquid per hour for each square foot of packing cross sectional area. It is possible to increase liquid flow by increasing reflux. However, as previously discussed, this also increases the vapor flow, which requires an increase in diameter, thus creating additional cross sectional area, and further reducing the liquid loading per square foot thereby defeating the purpose of the reflux increase. Typically the liquid loading in the Styrene Finishing Column is so low that structural packing is not used.
Historically, large diameter packed columns with low liquid loads are inefficient. Poor liquid distribution and also dry areas of packing result in lost efficiency. A recent analysis of an EB Recycle Column showed a 24% loss (from design) of efficiency. Efficiency drops as tower diameter increases and as liquid loading decreases in all packed columns per Klemas and Bonilla, Chemical Engineering Progress, July 1995, PP 27-44.
Styrene distillation towers using current technology are not very rate flexible. Rates may not be significantly increased due to maximum vapor flow, or be significantly decreased due to minimum liquid loading. Rates may be decreased by loading the tower with extra reflux to compensate for the reduction in product. However, this practice produces higher per pound energy costs.
Styrene towers are difficult and expensive to construct. A worldscale EB Recycle Column must be field constructed, a costly and time-consuming process. The EB Recycle Column is typically the single most expensive item on a styrene project and may represent 5 to 10% of the entire plant cost. The maximum capacity of a styrene facility has in the past been limited by the technology available for construction of an EB Recycle Column.
A need thus exists for new technology which will avoid most, if not all, of the problems, limitations, and high costs normally involved in the construction, operation, and maintenance of large scale distillation facilities for separation and purification of vinylaromatic monomers, such as styrene. This invention is deemed to fulfill this need in an efficient and effective manner.